Begonias & Powdery Mildew
What is Powdery Mildew in Begonias?
Powdery Mildew! The thing that strikes fear into the heart of every begonia collector! Oh no! Oh … wait. Every Autumn and Winter, we see people panicking in online forums about this dreaded white scourge, but is it really worth the mass hysteria?
Powdery mildew (PM) has many different strains, each affecting different genuses of plants. This means that if a begonia has PM, it is most likely the strain called Odium begoniae, which only affects begonias. While O. begoniae won't spread to other plant families, it will spread readily between your begonias.
PM infects plants via microscopic spores that are spread via air movement, however O. begoniae thrives in cooler, stagnant, humid environments. The definition of ‘cool’ will vary depending on the species of begonia, but generally warmer-growing species are more susceptible, as well as those with thin leaves. Most strains of PM have evolved to not kill a host, as dead tissue provides them nothing to survive on, so they are usually superficial infections, however if left untreated it will eventually result in defoliation leading to death.
Prevention
Prevention is better than cure. Taking steps to prevent PM from entering your collection will save you a lot of time and effort. This is best done by removing any blemished leaves on newly acquired plants and spraying with fungicide before adding it to your collection. When the nights start getting cooler in early Autumn, start preventative spraying with Eco-fungicide, which changes the pH of the leaf so that rogue PM spores are unlikely to germinate and infect the plant.
Complete prevention is not always possible and despite your best efforts it could still show up on one of your plants and if not caught early enough it can spread like wildfire. When it comes to begonias, incorrect cultural care is usually what will give PM a chance to proliferate. High humidity combined with stagnant, cold air is the perfect environment. Many growers experience this first in Autumn, when night time temperatures plummet. Lower temperatures result in higher humidity.
At the start of autumn, or when night time temps drop start regularly inspecting plants for early signs of PM and look at ways to heat your growing areas. Terrariums or tubs under lights will often retain enough heat through the night to prevent PM.
Larger spaces, however, will cool quickly overnight, so supplementary heating can help prevent PM: I use heat mats on the bottom of each begonia tent that are on the same timer as my lights, and the insulated tent materials allow the heat produced to be retained. This is enough to take the edge off and create less conducive conditions for PM to flourish. If you grow in a glass cabinet, like the popular Ikea Milsbo, glass will not insulate as well overnight, so you may need to ensure that heating elements remain on during the night.
But if PM spores are dispersed through air movement, what should you be doing with your fans? Over winter, I find that leaving fans on during the night will drop temperatures too much and could spread the spores at their peak growth time, so I turn them off. This is because my heat mats also turn off at night. I would always suggest having fans running during the day, however, because while standing water on begonia leaves will not cause PM (PM is actually dampened by water and will not spread if wet), it will create a colder, more humid environment overnight. Watering in the mornings will help to minimise this, too.
Overcrowding is another way to encourage the spread of PM, so ensure your plants have enough room to breathe. And remove any damaged or fallen leaves promptly.
Treatment
PM fungus populates the surface layer of leaves, stems, and petioles, so systemic fungicides are usually overkill and can have little effect except adding to the damage.
Remove heavily affected leaves by first spraying them with water to dampen the spores and avoid spreading them. Lightly affected leaves can be treated with a fungicide. There are a lot of natural methods around that many people swear by, but they often do little to control PM once it has actively taken hold. Personally, I personally prefer to bring out the big guns; my favourites are Rose Shield and Trifend, which knock out the spores on contact. Whatever you decide to use, test on one plant prior to drenching your whole collection so as to avoid burning those delicate leaves. Be mindful to rotate your fungicides to prevent the PM strain becoming resistant.
Cleaning the growing environment and tools after an infection is imperative, as dormant spores can spring to life the next time conditions are favourable.
Here are my main tips for you to take away from this:
Prevention is better than cure
Aim for temperature to not dip below 18c for more than a couple of hours
Turn fans off at night during the cooler months
Wet down any affected leaves prior to removal
Maintain good hygiene practices
Invest in a couple of good fungicides that are effective against strain Odium begoniae
Water earlier in the day to allow time for leaves to dry
So, let's not get our begonias in a twist over powdery mildew. Remember that a good growing environment and hygiene practices (and a great fungicide) can take the sting out of this time of year.